Ripple Formation /h3>

Name: Solakis
Status: parent
Grade: other
Location:  Great Britain

Question: My stepson raised an interesting question this morning
about why rain, when it falls in water, does it create a circular
ripple. My explanation is that the energy transferred from the
falling drop to the water is disbursed evenly around the raindrop
(as a pressure wave?) assuming that there is no obstruction. Could
it also be that the first initial contact with the water, creates a
type of contraction were the water around it collides inwards and
then reflects outwards as a wave/ripple?
---------------------------------------
Solakis,

Your first instinct of having the energy transfered equally in all 
directions is the more accepted interpretation. Think of it this way, 
the only reason energy would be transfered in a non-spherical fashion 
is if the different points beyond the point of impact are non-equal. 
For example, the air above the water is not a very good energy conduit 
of pressure when compared to the liquid water. As such, more energy is 
transmitted through the water then through the air above it. Also, if 
the water at the surface were sufficiently different in density or some 
other physical property then the energy might transmit more on the 
surface then on the bottom (Under normal conditions this is not true 
so we should expect energy transmittal to be like a half-sphere from 
the point of impact.) You already know this by including in your 
statement that "there is no obstruction". Again obstructions change 
the system and make the medium of transport unequal.

Greg (Roberto Gregorius)
====================================================================
You are pretty much right on. When a water droplet hits a water surface, the
kinetic energy contained in the droplet is transferred to the water, and
through viscosity is dissipated in the form of a wave. The wave spreads
outward from the source. The wave spreads outward by viscosity -- here
viscosity means the ability for one layer of molecules to exert a force on
the neighboring layer of molecules. As the droplet hits the water surface,
it drags neighboring molecules with it. They in turn drag neighboring
molecules with them. This creates the initial wave front. However,
eventually, the downward force of the droplet is counteracted by surface
tension and gravity, and the water molecules rebound upward. This upward
motion is spread outward by the same viscous mechanism. This up and down
repetition continues until the droplet's energy is fully dissipated. The
downward, then upward, motions create the wavelike pattern. The reason the
wave is circular is because the viscosity is the same throughout the water,
and so the wave has the same velocity in all directions. To have a
non-circular wave would require some non-uniform property of the water, or
some external force (like the obstruction you mention).

Hope this helps,
Burr Zimmerman
====================================================================